The implantation of prosthetic teeth to replace those lost to trauma or natural causes dates to at least the pre-Columbian era in the Americas wherein excavation of a Mayan burial site has identified skeletons with shells integrated into the mandible. More recent history, however, dates to approximately 1965 during which an experimental subject was implanted with a screw-like titanium device that in time became incorporated into the subject's bone.
Currently, the majority of dental implants are shaped like small, self-tapping screws, with either tapered or parallel sides. While those skilled in the art will understand that while a significant amount of preparation and skill is required in order to perform the procedure successfully, the procedure is conceptually simple. The orthodontic surgeon extracts the damaged tooth, screws in the implant, and after sufficient time has passed to allow the surgical site to heal and for bone to grow into and around the implant, a prosthetic tooth is threaded into the implant. Alternatively, in cases where the tooth and its root have been gone for a sufficiently long period of time such that the hole where the tooth root had been has filled with bone, a hole must first be drilled prior to carrying on the operations detailed above. Finally, where entire sections of dentition must be replaced, the orthodontic surgeon may instead opt to use an implant supported fixed bridge whereby multiple implants are emplaced in order to anchor dental bridgework.
Complications with this procedure arise, however, due to the two to six months required for osteogenesis and osteointegration (i.e., the bone in growth described above) to occur. During this healing period, the implant cannot withstand the impacts resulting from ordinary life, for example mastication or night-time tooth grinding. Hence, various protective strategies have been employed to ensure proper healing. Most commonly, a cover screw is threaded into the same hole which is later used for attaching the prosthetic tooth such that its head lies at or below the gum line so that the surrounding intact teeth will tend to protect the surgical site from impact. Unfortunately, all such methods are uncomfortable, and even painful to the patient. Perhaps more importantly, these methods generally cannot prevent movement of the implant during the up to six month healing process resulting in misalignment of the prosthesis potentially requiring reparative surgery. Therefore, there is a need for a way to speed healing, including osteogenesis and osteointegration.
The present invention is directed toward the electronic stimulation of bone to promote bone growth (osteogenesis) through or around a dental implant device in conjunction with an attached healing cap abutment functioning as a bone growth stimulator. In a technique similar to traditional dental implant procedures, the implant is first inserted into bone with or without a supplemental bone graft, or utilizing BMP, OPI, or other bone growth material including cement, biologicals, ceramics, glass, and the like. Thereafter, the bone growth stimulator of the present invention is attached to the dental screw implant whereupon the dental implant is activated by creating an electrical field around the entire screw and the surrounding tissues to promote osteogenesis of new bone and osteointegration as new bone growth forms into the surface of the implant. The device generates current ranging from about 5 μA to 50 μA to stimulate bone formation thereby reducing the healing time required before attachment of prosthetic dentition. After a period of time sufficient to ensure bone formation and osteointegration of the implant via, for example, a radiological study, a traditional dental crown or tooth is attached to the implant in lieu of the bone growth stimulator. In addition, it is contemplated that a “smart” tooth containing RFID technology and/or biological sensors may also be incorporated into the device.
It has long been known that the application of electric currents (electrical stimulation) can speed bone growth and healing. The present invention utilizes this phenomenon in a screw-type dental implant and bone growth stimulator for uses including expediting osteointegration and facilitating healing of the surgical site. Use of electrical current to stimulate bone growth has been known in the treatment of fractures, nonunion of bone and to hasten rates of bone fusion since at least the 1800s. Yasuda, in Japan in the 1950s, studied the effect of electricity in the treatment of fractures. E. Fukuda in “On the piezoelectric effect of bone”, J Physiol. Soc. Jpn. 12:1158-62, 1957, and Yasuda, J. Kyoto Med. Assoc. 4: 395-406, 1953 showed that electric signals could enhance fracture healing. Both direct current capacitative coupled electric fields and alternately pulsed electromagnetic fields affect bone cell activity in living bone tissue.
Bone has bioelectrical properties generated by naturally occurring stress potentials. When the bone is stressed, it will carry an electropositive charge on the convex side and an electronegative charge on the concave side. Accordingly, Wolff's Law dictates that bone will form new bone in areas of compression and will be resorbed in areas of tension. This biological response to stress in bone creates mechanically generated electrical fields or “strain related potentials.” Areas of active growth in bones carry an electronegative charge. When a bone fractures, the bone becomes electronegative at the fracture site. At the cellular level, it has been shown that osteoblasts are activated by electronegative charges. Research on the effects of electrical forces on bone cells with regard to bone formation and healing has demonstrated that bone healing can be hastened and enhanced by electricity. Studies have shown that by implanting an electrical stimulation device and applying an electrical current around the bone, bone formation is increased around the cathode (negative electrode) and decreased around the anode (positive electrode). Further research of the use of bone growth stimulators has shown that the optimal current for bone growth with electrical stimulation is between 5 and 20 μA.
K. S. McLeod and C. T. Rubin in “The effect of low frequency electrical fields on osteogenesis”, J. Bone Joint Surg. 74a:920-929, 1992, used varying sinusoidal fields to stimulate bone remodeling. These authors determined that extremely low frequency sinusoidal electric fields (smaller than 150 Hz) were effective in preventing bone loss and inducing bone formation. They also found strong frequency selectivity in the range of 15-30 Hz. Fitzsimmons et al. in “Frequency dependence of increased cell proliferation”, J Cell Physiol. 139(3):586-91, 1985, found a frequency specific increase in osteogenic cell proliferation at 14-16 Hz.
U.S. Pat. No. 5,292,252 issued Mar. 8, 1994 discloses a stimulator healing cap powered by a small internal battery. The cap can be reversibly attached to a dental implant, and stimulates bone growth and tissue healing by application of a direct current path or electromagnetic field in the vicinity of bone tissue surrounding the implant, after the implant is surgically inserted. Its implant, however, uses a traditional electrically conductive “titanium or titanium alloy” material such that particular attention must be paid to ensuring the device is not electrically shorted. Moreover, the design of the device is such that electrical stimulation is directed at the cortical bone at the surface of the mandible rather than the cancellous tissue where osteointegration primarily occurs.
Another dental device described in U.S. Pat. No. 4,027,392 issued Jun. 7, 1972 discloses an embodiment of a bionic tooth powered by a battery including an AC circuit. The microcircuitry indicated by its FIG. 3 is not shown as being incorporated within the cap. While its “battery 36” can be withdrawn once healing has been completed, most of the circuitry and associated electronics remains embedded in the patient, with the attendant increases risk of microbial infiltration therein and infection. Moreover, this device suffers from the same disadvantage as the '252 patent as stimulation is directed at the cortical layer of bone rather than at the cancellous tissues where the majority of the anchor body is located and electrical stimulation would be most effective.
Yet another related device is disclosed by in U.S. Pat. No. 5,738,521 issued Apr. 14, 1998 which describes a method for accelerating osteointegration of metal bone implants using AC electrical stimulation, with a preferably symmetrical 20 μA rms, 60 kHz alternating current signal powered by a small 1.5 V battery. This system is not a compact, self-powered stimulator cap, but is externally wired and powered.
Osteogenic devices are described in U.S. Pat. No. 6,605,089 issued Aug. 12, 2003 which discloses a self contained implant having a surgically implantable, renewable power supply and related control circuitry for delivering electrical current directly to an implant which is surgically implanted within the intervertebral space between two adjacent vertebrae. Electrical current is delivered directly to the implant and thus directly to the area in which the promotion of bone growth is desired.
U.S. Pat. No. 6,034,295 issued Mar. 7, 2000 discloses an implantable device with a biocompatible body having at least one interior cavity that communicates through at least one opening with the surrounding body so that tissue surrounding the implantable device can grow through the opening. Two or more electrodes are contained within the device having terminals for supplying a low-frequency electrical alternating voltage and at least one of which is located inside the cavity. U.S. Pat. No. 5,030,236 issued Jul. 9, 1991 discloses the use of electrical energy that relies upon radio frequency energy coupled inductively into an implanted coil to provide therapeutic energy. However, none of these devices perform satisfactory osteogenesis promotion, while leaving the implant member or stem essentially unchanged in appearance and mechanical properties.
The art that relates specifically to bone growth stimulation by small, self powered electrical means is very limited and most of the bone graft stimulation has been undertaken using power sources located outside the patient's body because of the problem that when the implant is self powered, the power short circuits against the metal screw or device.
Accordingly, the present invention is a self-powered osteogenesis-inducing dental screw-type implant that can generate electrical stimulation signals when used in conjunction with its